A wireless access system supports a sleep mode of a mobile station in order to minimize power consumption. A mobile station in a sleep mode is operates according to a sleep interval, which increases at a certain ratio. The sleep interval includes a listening window (e.g., listening interval) and a sleep window (e.g., sleep interval).
The listening interval is fixed by a sleep request message and a sleep response message. During the listening interval, a mobile station receives an indication of whether downlink traffic is being transmitted to the mobile station through a broadcast MAC management message (e.g., MOB-TRF-IND) sent from a base station (BS). The mobile station also receives an indication of whether ranging for maintaining uplink communications and a proper downlink coding type based on signal quality should be performed.
The sleep interval is a period during which the mobile station receives only a minimum downlink signal from a base station in order to reduce power consumption. During the sleep interval, the mobile station scans neighboring base stations for a handover and performs ranging for maintaining uplink communications and a proper downlink coding type according to signal quality.
The operation in the sleep mode includes transmitting and receiving a MAC management message, such as a sleep request (e.g., MOB-SLP-REQ) message, a sleep response (e.g., MOB-SLP-RSP) message or a broadcast downlink traffic indication (e.g., MOB-TRF-IND) message, between a mobile station and a base station.
FIG. 1 illustrates an exemplary sleep request message transmitted by a mobile station to a serving base station to request sleep mode.
Referring to FIG. 1, the MOB-SLP-REQ message is a type of MAC management message used for the sleep mode operation and includes a sleep interval and a listening interval.
FIG. 2 illustrates an exemplary sleep response message transmitted by the serving base station to the mobile station.
Referring to FIG. 2, the MOB-SLP-REQ message includes sleep mode-related information such as information regarding whether a sleep mode is approved, a sleep interval, a listening interval, or a sleep ID.
FIG. 3 illustrates an exemplary broadcast traffic indication message transmitted to the mobile station at a certain interval.
Referring to FIG. 3, upon receiving the MOB-TRF-IND message during a listening interval, the mobile station in the sleep mode determines whether to maintain the sleep mode, to terminate the sleep mode and receive downlink data, or to perform ranging during a sleep interval.
FIG. 4 is a signal flow diagram illustrating operation in the sleep mode.
Referring to FIG. 4, a mobile station generates a sleep request (e.g., MOB-SLP-REQ) message. The sleep request message includes values for an initial sleep interval (e.g., initial sleep window), a final sleep interval (e.g., final sleep window), and a listening interval. The mobile station then transmits the MOB-SLP-REQ message to a base station to request transition to a sleep mode. The base station transmits to the mobile station a sleep response (e.g., MOB-SLP-RSP) message in which the initial sleep interval, the final sleep interval, the listening interval, and a conversion start frame are set, thereby approving the transition to the sleep mode.
Accordingly, when a sleep mode conversion start time (e.g., M) has been reached, the mobile station maintains a sleep mode during the initial sleep interval (e.g., N1 frame). Thereafter, when the initial sleep interval expires, the mobile station receives a traffic indication (e.g., MOB-TRF-IND) message from the base station during a listening interval (e.g., L′). If no downlink traffic is being transmitted to the mobile station (e.g., a negative indication), the mobile station maintains a sleep mode during a period that is twice (e.g., 2*N1) as long as the initial sleep interval (e.g., N1).
The sleep interval is continuously increased according to the aforementioned equation such that the next sleep interval is set to be twice as long as the previous sleep interval. When the sleep interval reaches the final sleep interval (e.g., N2), the mobile station repeatedly uses the final sleep interval as the next sleep interval. The final sleep interval (e.g., sleep window) is set as follows according to the parameters shown in FIG. 2.Final sleep window=final sleep window*2final sleep window.
In contrast, if the mobile station is notified via a traffic indication (e.g., MOB-TRF-IND) message during the listening interval (e.g., a positive indication) that there is downlink traffic being transmitted to the mobile station, the mobile station terminates the sleep mode and receives the downlink traffic in a normal mode (e.g., awake mode). Furthermore, when receiving any uplink data, except for a ranging request (e.g., RNG-REQ) message, from the mobile station in sleep mode, the base station determines that the mobile station has terminated the sleep mode.
When there is no uplink signal from a specific mobile station during a certain period (e.g., a driving time of timer T27), the base station allocates an uplink slot to the mobile station, thereby allowing the mobile station to transmit to the base station a ranging request message for maintaining uplink synchronization (e.g., time, frequency and uplink transmission power) and for maintaining a proper coding rate according to signal quality. The base station, having received the ranging request message, sets a compensation value (e.g., time, frequency and uplink transmission power) for maintaining the uplink synchronization in a ranging response message and transmits the ranging response message to the mobile station.
FIG. 5 illustrates periodic ranging of a mobile station in the sleep mode.
Referring to FIG. 5, the base station sets, in a broadcast traffic indication (e.g., MOB-TRF-IND) message, a ranging operation time (e.g., ranging offset) and an information regarding whether or not the ranging is performed, and transmits the MOB-TRF-IND message to the mobile station in a sleep mode. The mobile station in the sleep mode receives the MOB-TRF-IND message during a listening interval.
Thereafter, when the ranging time has been reached, the mobile station transmits a ranging request (e.g., RNG-REQ) message to the base station using the allocated uplink slot, and the base station transmits a ranging response (e.g., RNG-RSP) message to the mobile station. The base station allocates an uplink link bandwidth (e.g., UL BW) for a periodic RNG-REQ. Accordingly, the mobile station receives the ranging response (e.g., RNG-RSP) message, adjusts the uplink synchronization with the base station, and maintains the sleep mode.
If there is a need for the mobile station in a sleep mode to perform periodic ranging during a sleep interval, the base station sets, in a broadcast MOB-TRF-IND message, a ranging operation time and information regarding whether the ranging is performed, and transmits the MOB-TRF-IND message to the mobile station. Thereafter, when recognizing that there is downlink traffic or that there is no ranging operation from the received MOB-TRF-IND message during the listening interval, the mobile station maintains a sleep mode during a sleep interval that is twice as long as a present sleep interval.
FIG. 6 illustrates a relationship between a sleep ID (SLPID) bitmap and a sleep ID in a traffic indication message.
Referring to FIG. 6, the format of the MOB-TRF-IND message is the same as that of FIG. 3.
For example, assuming that an SLPID bitmap of the MOB-TRF-IND is the same as that shown in FIG. 6, SLPID mobile stations that should perform periodic ranging are indicated. Assuming a number of mobile stations performing periodic ranging is seven, a ranging (e.g., ranging frame offset) of each mobile station is set as follows.
Ranging frame offset 1: frame offset of SLPID 2
Ranging frame offset 2: frame offset of SLPID 8
Ranging frame offset 3: frame offset of SLPID 9
Ranging frame offset 4: frame offset of SLPID 12
Ranging frame offset 5: frame offset of SLPID 15
Ranging frame offset 6: frame offset of SLPID 18
Ranging frame offset 7: frame offset of SLPID 20
Therefore, a single mobile station may perform only one periodic ranging during the next sleep interval after a listening interval. Only one periodic ranging may be performed by the single mobile station because, while the SLPID bitmap may indicate whether or not the ranging is performed, the SLPID bitmap may not indicate the number of times that ranging is to be performed.
In the related art, the base station performs periodic ranging using a specific timer (e.g., T27). If the base station cannot receive any uplink signal from a specific mobile station before the timer expires, the base station allocates an uplink slot to a mobile station to allow the mobile station to transmit a ranging request message to the base station and to perform the ranging operation. If the base station receives the uplink signal from the specific mobile station before the corresponding timer expires, the timer is reset. As described above, the sleep interval of a sleep mode increases exponentially from an initial sleep interval to a final sleep interval. The final sleep interval (e.g., sleep window) is set through a predetermined formula.
FIG. 7 illustrates a relationship between a periodic ranging period and a sleep interval.
Referring to FIG. 7, if a sleep interval is longer than a periodic ranging cycle, the mobile station is requested to perform periodic ranging two or more times during a corresponding sleep interval. However, in the related art, periodic ranging with respect to a single mobile station may not be performed more than once during a sleep interval, which creates problems because periodic ranging during the sleep mode may not be provided.